chapter 14

property is a public side walk or alley, local building codes may permit such footings ... But when the adjoining property is privately owned, the footings must be.
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CHAPTER 14 SHALLOW FOUNDATION III: COMBINED FOOTINGS AND MAT FOUNDATIONS 14.1

INTRODUCTION

Chapter 12 has considered the common methods of transmitting loads to subsoil through spread footings carrying single column loads. This chapter considers the following types of foundations: 1. Cantilever footings 2. Combined footings 3. Mat foundations When a column is near or right next to a property limit, a square or rectangular footing concentrically loaded under the column would extend into the adjoining property. If the adjoining property is a public side walk or alley, local building codes may permit such footings to project into public property. But when the adjoining property is privately owned, the footings must be constructed within the property. In such cases, there are three alternatives which are illustrated in Fig. 14.1 (a). These are 1. Cantilever footing. A cantilever or strap footing normally comprises two footings connected by a beam called a strap. A strap footing is a special case of a combined footing. 2. Combined footing. A combined footing is a long footing supporting two or more columns in one row. 3. Mat or raft foundations. A mat or raft foundation is a large footing, usually supporting several columns in two or more rows. The choice between these types depends primarily upon the relative cost. In the majority of cases, mat foundations are normally used where the soil has low bearing capacity and where the total area occupied by an individual footing is not less than 50 per cent of the loaded area of the building. When the distances between the columns and the loads carried by each column are not equal, there will be eccentric loading. The effect of eccentricity is to increase the base pressure on the side

585

586

Chapter 14

of eccentricity and decrease it on the opposite side. The effect of eccentricity on the base pressure of rigid footings is also considered here. Mat Foundation in Sand A foundation is generally termed as a mat if the least width is more than 6 meters. Experience indicates that the ultimate bearing capacity of a mat foundation on cohesionless soil is much higher than that of individual footings of lesser width. With the increasing width of the mat, or increasing relative density of the sand, the ultimate bearing capacity increases rapidly. Hence, the danger that a large mat may break into a sand foundation is too remote to require consideration. On account of the large size of mats the stresses in the underlying soil are likely to be relatively high to a considerable depth. Therefore, the influence of local loose pockets distributed at random throughout the sand is likely to be about the same beneath all parts of the mat and differential settlements are likely to be smaller than those of a spread foundation designed for the same soil

Property line

Mat

Strap footin

\— Combined footing /

(a) Schematic plan showing mat, strap and combined footings

T T T T T T T T T T T T T T I q

(b) Bulb of pressure for vertical stress for different beams Figure 14.1

(a) Types of footings; (b) beams on compressible subgrade

Shallow Foundation III: Combined Footings and Mat Foundation

587

pressure. The methods of calculating the ultimate bearing capacity dealt with in Chapter 12 are also applicable to mat foundations. Mat Foundation in Clay The net ultimate bearing capacity that can be sustained by the soil at the base of a mat on a deep deposit of clay or plastic silt may be obtained in the same manner as for footings on clay discussed in Chapter 12. However, by using the principle of flotation, the pressure on the base of the mat that induces settlement can be reduced by increasing the depth of the foundation. A brief discussion on the principle of flotation is dealt with in this chapter. Rigid and Elastic Foundation The conventional method of design of combined footings and mat foundations is to assume the foundation as infinitely rigid and the contact pressure is assumed to have a planar distribution. In the case of an elastic foundation, the soil is assumed to be a truly elastic solid obeying Hooke's law in all directions. The design of an elastic foundation requires a knowledge of the subgrade reaction which is briefly discussed here. However, the elastic method does not readily lend itself to engineering applications because it is extremely difficult and solutions are available for only a few extremely simple cases.

14.2 SAFE BEARING PRESSURES FOR MAT FOUNDATIONS ON SAND AND CLAY Mats on Sand Because the differential settlements of a mat foundation are less than those of a spread foundation designed for the same soil pressure, it is reasonable to permit larger safe soil pressures on a raft foundation. Experience has shown that a pressure approximately twice as great as that allowed for individual footings may be used because it does not lead to detrimental differential settlements. The maximum settlement of a mat may be about 50 mm (2 in) instead of 25 mm as for a spread foundation. The shape of the curve in Fig. 13.3(a) shows that the net soil pressure corresponding to a given settlement is practically independent of the width of the footing or mat when the width becomes large. The safe soil pressure for design may with sufficient accuracy be taken as twice the pressure indicated in Fig. 13.5. Peck et al., (1974) recommend the following equation for computing net safe pressure, qs = 2lNcorkPa

(14.1)

for 5 < Ncor < 50 where Ncor is the SPT value corrected for energy, overburden pressure and field procedures. Eq. 14.1 gives qs values above the water table. A correction factor should be used for the presence of a water table as explained in Chapter 12. Peck et al., (1974) also recommend that the qs values as given by Eq. 14.1 may be increased somewhat if bedrock is encountered at a depth less than about one half the width of the raft. The value of N to be considered is the average of the values obtained up to a depth equal to the least width of the raft. If the average value of N after correction for the influence of overburden pressure and dilatancy is less than about 5, Peck et al., say that the sand is generally considered to be too loose for the successful use of a raft foundation. Either the sand should be compacted or else the foundation should be established on piles or piers.

588

Chapter 14

The minimum depth of foundation recommended for a raft is about 2.5 m below the surrounding ground surface. Experience has shown that if the surcharge is less than this amount, the edges of the raft settle appreciably more than the interior because of a lack of confinement of the sand.

Safe Bearing Pressures of Mats on Clay The quantity in Eq. 12.25(b) is the net bearing capacity qm at the elevation of the base of the raft in excess of that exerted by the surrounding surcharge. Likewise, in Eq. 12.25(c), qna is the net allowable soil pressure. By increasing the depth of excavation, the pressure that can safely be exerted by the building is correspondingly increased. This aspect of the problem is considered further in Section 14.10 in floating foundation. As for footings on clay, the factor of safety against failure of the soil beneath a mat on clay should not be less than 3 under normal loads, or less than 2 under the most extreme loads. The settlement of the mat under the given loading condition should be calculated as per the procedures explained in Chapter 13. The net safe pressure should be decided on the basis of the permissible settlement.

14.3

ECCENTRIC LOADING

When the resultant of loads on a footing does not pass through the center of the footing, the footing is subjected to what is called eccentric loading. The loads on the footing may be vertical or inclined. If the loads are inclined it may be assumed that the horizontal component is resisted by the frictional resistance offered by the base of the footing. The vertical component in such a case is the only factor for the design of the footing. The effects of eccentricity on bearing pressure of the footings have been discussed in Chapter 12.

14.4

THE COEFFICIENT OF SUBGRADE REACTION

The coefficient of subgrade reaction is defined as the ratio between the pressure against the footing or mat and the settlement at a given point expressed as

where

& y = coefficient of subgrade reaction expressed as force/length3 (FZr3), q = pressure on the footing or mat at a given point expressed as force/length2 (FZr2), S = settlement of the same point of the footing or mat in the corresponding unit of length.

In other words the coefficient of subgrade reaction is the unit pressure required to produce a unit settlement. In clayey soils, settlement under the load takes place over a long period of time and the coefficient should be determined on the basis of the final settlement. On purely granular soils, settlement takes place shortly after load application. Eq. (14.2) is based on two simplifying assumptions: 1 . The value of k^ is independent of the magnitude of pressure. 2. The value of & s has the same value for every point on the surface of the footing. Both the assumptions are strictly not accurate. The value of ks decreases with the increase of the magnitude of the pressure and it is not the same for every point of the surface of the footing as the settlement of a flexible footing varies from point to point. However the method is supposed to

Shallow Foundation III: Combined Footings and Mat Foundation

589

give realistic values for contact pressures and is suitable for beam or mat design when only a low order of settlement is required. Factors Affecting the Value of ks Terzaghi (1955) discussed the various factors that affect the value of ks. A brief description of his arguments is given below. Consider two foundation beams of widths Bl and B2 such that B2 = nB{ resting on a compressible subgrade and each loaded so that the pressure against the footing is uniform and equal to q for both the beams (Fig. 14. Ib). Consider the same points on each beam and, let >>! = settlement of beam of width B\ y2 = settlement of beam of width B2 q

Hence

Ir



**i ~

y\

Qt"lH ana KI " 7

q —

s2 ~

./2

If the beams are resting on a subgrade whose deformation properties are more or less independent of depth (such as a stiff clay) then it can be assumed that the settlement increases in simple proportion to the depth of the pressure bulb. Then and

y2 = nyl ks2 —3—±?L-k l i ~ nv ~ v B ~ B '

A general expression for ks can now be obtained if we consider B{ as being of unit width (Terzaghi used a unit width of one foot which converted to metric units may be taken as equal to 0.30 m). Hence by putting B} = 0.30 m, ks = ks2, B = B2, we obtain k,=Q3-j-

(14.4)

where ks is the coefficient of subgrade reaction of a long footing of width B meters and resting on stiff clay; ksl is the coefficient of subgrade reaction of a long footing of width 0.30 m (approximately), resting on the same clay. It is to be noted here that the value of ksl is derived from ultimate settlement values, that is, after consolidation settlement is completed. If the beams are resting on clean sand, the final settlement values are obtained almost instantaneously. Since the modulus of elasticity of sand increases with depth, the deformation characteristics of the sand change and become less compressible with depth. Because of this characteristic of sand, the lower portion of the bulb of pressure for beam B2 is less compressible than that of the sand enclosed in the bulb of pressure of beam Bl. The settlement value y2 lies somewhere between yl and nyr It has been shown experimentally (Terzaghi and Peck, 1948) that the settlement, y, of a beam of width B resting on sand is given by the expression

2B ^ where y{ = settlement of a beam of width 0.30 m and subjected to the same reactive pressure as the beam of width B meters.

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Chapter 14

Hence, the coefficient of subgrade reaction k^ of a beam of width B meters can be obtained from the following equation 5 + 0.30

= k..

(14.6)

where ks{ = coefficient of subgrade reaction of a beam of width 0.30 m resting on the same sand. Measurement of Ars1 A value for £ v l for a particular subgrade can be obtained by carrying out plate load tests. The standard size of plate used for this purpose is 0.30 x 0.30 m size. Let k} be the subgrade reaction for a plate of size 0.30 x 0.30 rn size. From experiments it has been found that & ?1 ~ k{ for sand subgrades, but for clays ksl varies with the length of the beam. Terzaghi (1955) gives the following formula for clays *5i = *i

L + 0.152 (14.1 a)

where L = length of the beam in meters and the width of the beam = 0.30 m. For a very long beam on clay subgrade we may write

Procedure to Find Ars For sand 1. Determine k^ from plate load test or from estimation. 2. Since &sd ~ k { , use Eq. (14.6) to determine ks for sand for any given width B meter. For clay 1 . Determine k{ from plate load test or from estimation 2. Determine & y , from Eq. (14.7a) as the length of beam is known. 3. Determine ks from Eq. (14.4) for the given width B meters. When plate load tests are used, k{ may be found by one of the two ways, 1. A bearing pressure equal to not more than the ultimate pressure and the corresponding settlement is used for computing k{ 2. Consider the bearing pressure corresponding to a settlement of 1.3 mm for computing kr Estimation of Ar1 Values Plate load tests are both costly and time consuming. Generally a designer requires only the values of the bending moments and shear forces within the foundation. With even a relatively large error in the estimation of kr moments and shear forces can be calculated with little error (Terzaghi, 1955); an error of 100 per cent in the estimation of ks may change the structural behavior of the foundation by up to 15 per cent only.

Shallow Foundation III: Combined Footings and Mat Foundation

591

/TI values for foundations on sand (MN/m3)

Table 14.1a Relative density SPT Values (Uncorrected)

Loose 30

Soil, dry or moist Soil submerged

15 10

45 30

175 100

Table 14.1b

/:1 values for foundation on clay

Consistency

Stiff

Very stiff

Hard

cu (kN/m 2 ) *, (MN/m3)

50-100 25

100-200 50

>200 100

Source: Terzaghi (1955) In the absence of plate load tests, estimated values of kl and hence ks are used. The values suggested by Terzaghi for k\ (converted into S.I. units) are given in Table 14.1.

14.5

PROPORTIONING OF CANTILEVER FOOTING

Strap or cantilever footings are designed on the basis of the following assumptions: 1 . The strap is infinitely stiff. It serves to transfer the column loads to the soil with equal and uniform soil pressure under both the footings. 2. The strap is a pure flexural member and does not take soil reaction. To avoid bearing on the bottom of the strap a few centimeters of the underlying soil may be loosened prior to the placement of concrete. A strap footing is used to connect an eccentrically loaded column footing close to the property line to an interior column as shown in Fig. 14.2. With the above assumptions, the design of a strap footing is a simple procedure. It starts with a trial value of e, Fig. 14.2. Then the reactions Rl and R2 are computed by the principle of statics. The tentative footing areas are equal to the reactions R{ and R2 divided by the safe bearing pressure q . With tentative footing sizes, the value of e is computed. These steps are repeated until the trial value of e is identical with the final one. The shears and moments in the strap are determined, and the straps designed to withstand the shear and moments. The footings are assumed to be subjected to uniform soil pressure and designed as simple spread footings. Under the assumptions given above the resultants of the column loads Ql and Q2 would coincide with the center of gravity of the two footing areas. Theoretically, the bearing pressure would be uniform under both the footings. However, it is possible that sometimes the full design live load acts upon one of the columns while the other may be subjected to little live load. In such a case, the full reduction of column load from